Motherboard, CPU, RAM, and video card. With $450, you can get this:
Abit NF7-S 2.0: $95 shipped
Athlon XP 2500+ Retail (includes hsf): $83 shipped
Kingston 512mb HyperX PC2700: $95 shipped
ATI Radeon 9600 XT 128mb: $151 shipped
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Total: $424
Those are all prices from Newegg. It'll get you up to speed. It won't be the best, but it will more than double your current performance. I'd also get an Antec/Enermax 350W or better PSU (no other brands, as most overrate the output). That'll cost you $50-$100, but remember you'll be able to sell your old CPU, board, RAM, and GF3 and make at least $75 back.
The 2500+ is a very nice chip. I build the machine in my sig 8 months ago, and I'm still extremly satisfied with the performance. NFSU, FarCry, and UT2004 are all running great. I'm even running at 4x AA and 16x AF in all games except for FarCry (the demo has some issues), and it's still very smooth. The system is by no means "obselete". It'll get by for another year at least. If you'd like to build a machine that'll last longer, you're going to have to save up some more and wait for the Athlon 64 to mature and drop in price. Right now you'd be hard pressed to even get the slowest A64 3000+ and a motherboard for $450 shipped -- and you'd still need some new PC3200 DDR and a better video card.
The 9800 Pro is by no means going out of style! Right now the bottleneck in games for me is my CPU. When I'm able to run the latest games (DX8-9 games; UT2k4) at very smooth frame rates with 4x AA and 16x AF at 1024x768x32 and settings at maximum quality, it's evident that the video card has a LOT of life left in it! A year later, I bet I'll be able to throw the 9800 Pro in a system with a faster CPU + mobo, turn off the AA and AF, and still play new games at high frame rates. 3DMark 2001 proves that, too...my stock score on this system is 15,615. I've seen people with 9800 Pros on Athlon 64 3200+'s and the high end P4's scoring almost 20,000 stock. So, the 9800 Pro is held back by current processors.
You may want to wait until late March-April (only 2 months), because there should be some major releases then. ATI's next GPU and AMD's next A64's should be making their debut. That stuff will be expensive, but it'd knock the prices down on the faster stuff we have right now.
obviously, sound is great on your system (audigy 2), but that nforce is just as good, so you're in a dilemma here...
NO! I've used numerous boards with the Realtek ALC650 audio chip (nForce 2 audio), and my two primary computers in my home have SoundStorm. I hate it. I'm not gonna go into detail, but an audiophile like myself will be able to instantly point out the poor sound quality. I'm using an Audigy 2 ZS, and the sound is much, much better. I gave SoundStorm a shot and hated it so much I put my old SB Live 5.1 back in the computer in under 24 hours.